High pressure reactor polymerization plants convert relatively low cost olefin monomers (generally ethylene, optionally in combination with one or more comonomers such as vinyl acetate) into valuable polyolefin products. Such processes using oxygen or organic free-radical initiators, in particular peroxide initiators, are known in the art and have been used in industry for a long time. The polymerization takes place at relatively high temperatures and pressures and is highly exothermic. The resulting polymer is a low density polyethylene (LDPE), optionally containing comonomers.
High pressure polymerization processes are carried out in autoclave or tubular reactors. In principle, the autoclave and the tubular polymerization processes are very similar, except for the design of the reactor itself. The plants generally use two main compressors arranged in series, each with multiple stages, to compress the monomer feed. A primary compressor provides an initial compression of the monomer feed, and a secondary compressor increases the pressure generated by the primary compressor to the level at which polymerization takes place in the reactor, which is typically about 210 to about 320 MPa for a tubular reactor and about 120 to about 200 MPa for an autoclave reactor.
The interstage pressure of the secondary compressor is high enough to result in thermal polymerization, which causes a high density, high molecular weight fouling layer of polymer to form on the inner surfaces of the compressor piping. Thermal polymerization is initiated by the presence of radicals and progresses at a higher rate when temperatures are higher and more radicals are present. Fouling can lead to a complete plugging of gas flow lines in the remainder of the process and unfavorably high pressure drop, which can lead to reduced compressor throughput, poor pumping efficiency, and damage in the secondary compressor. Fouling also increases the risk of decompositions reactions, which occur when active radicals are trapped in the polymer layer.
Removal of the fouling layer requires the use of either mechanical or chemical cleaning. Typically, a shutdown of more than twenty hours is required. Additionally, there is a risk that portions of the polymer layer may come loose, especially during start up, and form plugs in the system, requiring dismantling of the piping for complete removal.
Background references include US 2003/008982, US 2007/032614, and EP 0 144 716 A.
There is a need for a processes and systems for producing low density polyethylene that overcome the possibility for premature polymerization and fouling in the compressor, without significantly changing compressor operating conditions or requiring equipment to be shut down regularly for cleaning. There is also a need for piping designs that allow piping to be easily and quickly dismantled for cleaning to minimize shut down and downtime costs.